Just bought this ebay power regulator and connected 17vac to the ac input everthing ok then when I connected the ground from the supply
the rectifier started smokin. I used a variac for power supply.
When I disconnected the ground, works good.

Just bought this ebay power regulator and connected 17vac to the ac input everthing ok then when I connected the ground from the supply
the rectifier started smokin. I used a variac for power supply.
When I disconnected the ground, works good.

Why would connecting the ground cook the rectifier ???

Click to expand...

What do you mean ground from the supply? If it is an AC source, both lines should be connected to the AC inputs (1 and 2), you should not connect the AC ground to the DC ground!

The ground on the variac. like all 120vac supplies you and a hot, neutral and "ground" wire.

Unless you use a isolation transformer .

Click to expand...

AC ground wire does not connect to the DC ground, the "earth" ground can be connected to the power supply chassis (metal frame) in either case for safety.

For circuitry in situations where significant Earth ground currents can flow (like on electric-powered Subway trains), you will want to isolate all logic grounds from Earth/chassis grounds, and maybe only connect the two occasionally in the devices with a charge bleeder resistor like 1Meg in parallel with a small high-frequency cap like 1000pF.

Thank you Bmoris, that was the answer i was looking for.
""
For circuitry in situations where significant Earth ground currents can flow (like on electric-powered Subway trains), you will want to isolate all logic grounds from Earth/chassis grounds, and maybe only connect the two occasionally in the devices with a charge bleeder resistor like 1Meg in parallel with a small high-frequency cap like 1000pF
""

I need a fuller understanding of the types of "grounds " isolated vs earth
dc grounds vs ac grounds . and the relationship to oscilloscopes ground.
I have to remimber the oscope ground is a direct ac earth type.

The (earth) ground (third pin on the plug) and neutral (LARGER of the 2 blades) (when all wired correctly) are connected, usually back at the breaker panel. Neutral is thus close to earth ground in potential, only different by the drop from current is the wire.

Let me try to answer what I think you're asking. Feel free to ask for clarification if I don't get at what you're really asking.

The ground on your variac is a safety ground. It eventually connects to the AC neutral at some point in your house wiring. This is why connecting it to you circuit "ground" causes a problem. As someone else pointed out, you are shorting out a diode in your bridge.

The "ground" in your circuit is a "common" point to reference voltages to. You can not connect it to the AC safety ground for the reasons mentioned above.

In many designs there is further isolation. For example, the rectified DC voltage fron the bridge might feed an isolated flyback regulator. A flyback is a switching power converter with a transformer for isolation. In this case the "ground" on the output side of the flyback converter circuit is completely isolated from AC mains and it is OK to connect the AC safety ground it.

As you can see, the concept of ground can be confusing. Hope this helps.

Just to make sure it's clear, you cannot connect the bridge output common (ground) to the common of the input to the bridge, which is what you do when you connect the output of an AC powered bridge to the AC ground. That connects two of the diodes directly across the power which makes them act as a fuse, as you observed.